ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 21 
(1) The first cleavage spindle is eccentrie toward the posterior pole of the egg, 
and the median plane of the future embryo is marked out by the bilateral symmetry 
of the unsegmented egg (p. 6). 
(2) The plane of the first cleavage coincides with the plane of bilateral sym- 
metry, and therefore divides the egg into right and left halves (p. 6). 
(3) The second cleavage plane is transverse to the long axis of the embryo 
and separates two large anterior cells from two small posterior ones (p. 7). 
(4) The intersection of these two planes marks the vertical axis of the egg; one 
end of this axis corresponds to the middle of the dorsal, the other to the middle of 
the ventral face of the gastrula (p. 7). 
(5) The third cleavage separates 4 larger dorsal cells from 4 smaller ventral 
ones (p. 7); the latter are ectodermal, the former ** mixed." 
(6) At the fourth cleavage these 8 cells give rise to 16; 8 ventral cells, all 
ectodermal, and 8 dorsal cells, 6 of which are mixed, and 2, which are smaller than 
any of the others and lie at the posterior pole, ectodermal (p. 8). 
7) By division these 16 cells give rise to 32; 16 ventral cells, all ectodermal 
and 16 dorsal cells, 4 ectodermal derived from the 2 posterior ectoderm cells of the 
previous stage, 6 ectodermal derived from the 6 mixed cells, 4 endodermal and 2 
still mixed. With regard to the identification of the dorsal and ventral faces at 
this stage they say :—“Ou bien les cellules ectodermique forment ensemble une 
calotte appliquée par sa concavité contra les globes endodermiques et mixtes (comme 
dans fig. 10, c), ou bien c'est le contraire qui a lieu, les globes endodermiques et 
mixtes s'étalent en surface de facon a constituer ensemble une calotte moulée sur 
l'ectoderme (fig, 9, c)." 
(8) At the next stage there are 44 cells; 32 ectodermal, easily recognized by 
their transparency, and 12 other cells very much larger. The ectodermal cap is 
notably extended and tends to envelope the endoderm. 
From this stage onward there is no question as to the identification of the dor- 
sal and ventral faces or the anterior or posterior ends. As will presently appear, 
my work, like that of Chabry (1887), entirely confirms the orientation adopted by 
Van Beneden and Julin, though I cannot agree with them as to the fate of certain 
individual cells. 
| ; 2. Seeliger’s System. 
Seeliger’s (1885) later work was much less detailed and satisfactory with 
regard to the orientation of the early cleavage stages, as Castle has shown. His 
principal conclusions as to orientation are : 
(1) The first cleavage plane coincides with the median plane of the embryo, 
but neither anterior nor posterior, dorsal nor ventral can be recognized at this 
stage (p. 48). 
(2) The second cleavage divides the egg into two smaller anterior cells and 
two larger posterior ones (p. 48). 
(3) The third cleavage separates 4 dorsal endodermal cells from 4 ventral ecto- 
dermal ones; the two posterior ventral cells are larger than any of the others. 
Structurally all these cells are alike (p. 49). 
